The present invention relates generally to the field of wavelength division multiplexing within optical transmission systems, and more particularly to the field of wavelength division multiplexing having bidirectional transmission for interleaved optical signals amplified with rare-earth doped fiber amplifiers.
Optical transmission systems often includes optical amplifiers in their transmission paths to avoid excessive attenuation of a transmitted optical signal. The optical amplifier can be a length of optical fiber doped with a rare-earth element, for example erbium. These rare-earth-doped fiber amplifiers provide amplification of a characteristic transmission signal bandwidth when they are simultaneously stimulated or pumped with a characteristic pump wavelength. For fiber amplifiers doped with erbium, the characteristic pump wavelength is generally either 980 nm or 1480 nm, which results in a stimulated emission spectrum for the amplifier across a band of about 1530-65 nm. Therefore, the erbium-doped fiber amplifier will amplify transmission signals passing through it at these wavelengths. It is also known, e.g., from Electronics Letters, Vol. 26, No. 20, Sep. 27, 1990, p. 1645-46, that an erbium doped fiber amplifier can provide amplification in the 1570-1610 nm wavelength band, by appropriate selection of pumping conditions, active fiber doping and length. Wavelength-division-multiplexing (WDM) systems that transmit a plurality of information signals as modulated channels along a single optical path must use channel wavelengths that correspond with the stimulated emission spectrum particular to the erbium-doped fiber amplifier. In general, the erbium-doped fiber amplifier will amplify transmission signals passing through it at wavelengths in an extended band of about 1525-1610 nm or in sub-bands of the extended band. The following description will refer to a band of 1530-1565 nm. However, by making obvious changes the skilled in the art can apply the teaching of the invention to an extended band, for example from 1525 nm to 1610 nm, to sub-bands of the extended band or to other wavelength bands, as needed, in particular if active substances other than erbium are used.
In addition to generating stimulated emission due to the introduction of a characteristic pump wavelength, rare-earth doped fiber amplifiers also tend to generate unwanted amplified spontaneous emission (ASE). ASE, when subject to a high gain within the amplifier, contributes a substantial light level at the output of the amplifier and can saturate the amplifier output. Moreover, the ASE is nearly proportional to the amplifier gain, and therefore, the ASE spectrum is similar to the gain spectrum.
ASE also causes problems more specific to WDM systems. As mentioned, WDM systems carry a plurality of channels of modulated information over a common transmission medium, and when erbium-doped fiber amplifiers are used, generally have a carrier wavelength between about 1530 nm and 1565 nm. When the number of channels in a WDM system becomes dense, e.g. equals or exceeds sixteen (16), the wavelength spacing between the channels becomes practically small. As the spacing decreases, potential problems in differentiating between the channels arise, as do problems with increased crosstalk and decreased signal-to-noise ratio.
FIG. 1 illustrates the representative spectra of ASE noise generated by a typical erbium-doped fiber amplifier. The curve 100 in FIG. 1 depicts the ASE noise, which is similar to the stimulated emission spectra for the fiber amplifier. Signals 110 and 120 represent two generic wavelengths of a WDM system that are centered at predetermined wavelengths within the bandwidth 130 of an erbium-doped fiber amplifier in the system, which would span about 1530-65 nm. As shown in FIG. 1, the ASE noise creates a signal-to-noise ratio 140 for the WDM channels.
Several patents and publications have addressed techniques for removing ASE noise in a fiber amplifier system. U.S. Pat. No. 5,260,823 to Payne et al., for example, discloses particular advantages in locating a filter within the length of a fiber amplifier rather than at its end to remove ASE noise. The '823 patent states that an optical band-stop filter can be incorporated in the fiber at appropriate points using, for instance, thin colored-glass filters, Fabry-Perot filters, and Bragg filters. U.S. Pat. No. 5,283,686 to Huber discloses an arrangement having a circulator and a Bragg grating coupled to an erbium-doped fiber amplifier. A desired signal and undesired ASE enter a first port of the circulator from the fiber amplifier, and a Bragg grating attached to the second port of the circulator reflects the desired signal and allows the ASE to pass. According to the '686 patent, the desired signal returns to the circulator and exits from a third circulator port.
EP 729,248 discloses a bidirectional system for multichannel optical fiber communications. FIG. 2 in EP 729,248 illustrates bidirectional amplifier for interleaved channels f1, f2, f3, and f4, where f1 and f3 propagate in one direction and f2 and f4 propagate in the opposite direction. Channels f1 and f3 travel west to east in FIG. 2 of EP 729,248, rotate through circulator 20, are reflected by Bragg gratings 28 and 29, respectively, pass through amplifier 22, rotate through circulator 21, and exit from the system. Channels f2 and f4 travel in the opposite direction but interact with circulator 21, Bragg gratings 30 and 31, amplifier 23, and circulator 20. Bragg gratings 28-31 are used as filters in the bidirectional system but filter the channels as they enter the amplifier structure. As a result, the Bragg gratings in EP 729,248 do not filter ASE introduced by amplifiers 22 and 23.
Barnard et al., "Bidirectional Fiber Amplifiers," IEEE Photonics Tech. Ltrs., Vol. 4, No. 8, pp. 911-13 (1992) discloses a bidirectional erbium-doped fiber amplifier module that uses directional couplers to separate eastbound and westbound signals for amplification in separate fiber amplifiers. This paper recognizes that multiple reflection-induced relative intensity noise, e.g. due to Rayleigh back scattering, may lead to a power penalty for a bidirectional amplifier module. For direct digital detection at a desired bit error rate of 10.sup.-9, this power penalty is disclosed as: EQU penalty=-5 log [1-144 R.sub.eff.sup.2 ] (1)
where the effective reflectance R.sub.eff equals R.sub.1 R.sub.2 /2 for discrete reflections with intensity reflection coefficients R.sub.1 and R.sub.2, while R.sub.eff equals R.sub.bs /.sqroot.2 for Rayleigh back scattering, with R.sub.bs .apprxeq.32 dB for fibers longer than 20 km. If an unisolated optical amplifier with gain G is located between the reflections, the effective reflectance increases to GR.sub.eff. To help minimize the power penalty, the paper discloses that non-overlapping optical bands, e.g. 1525-35 nm and 1550-60 nm, could be assigned to the signals in opposite directions by adding narrow band pass optical filters to each unidirectional path in the bidirectional amplifier module.
Applicants have discovered that with the increased density of channels in WDM systems and the use of a bidirectional architecture using interleaved channels or interleaved packets of channels, the efficient removal of ASE noise, of other noise reflections between the channels of the system and of the echo of the channel themselves, due to reflection or back scattering, has a heightened importance to enabling a close channel-to-channel spacing for the system, for given spectral characteristics of the available wavelength selective components used to separate the signals at the various wavelengths.
Applicants have further found that an arrangement of optical circulators and Bragg gratings with rare-earth doped fiber amplifiers can provide a compact and practical apparatus for amplifying interleaved, bidirectional channels or packets of channels while removing unwanted ASE between them and protecting the system from interferometric noise due to unwanted reflections at fiber interfaces or due to Rayleigh back scattering in transmission fibers.